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West Nile virus is a mosquito-borne virus first identified in the West Nile region of Africa in 1937. Since then the virus has caused outbreaks of disease in Africa, Asia, Eastern Europe and the Middle East, but it did not appear in the United States until 1999. After first being discovered in birds and people in the metropolitan New York City area, it spread westward across the US and into Canada and Mexico. In October, 2006, West Nile virus was detected in King County for the first time when 6 birds and a horse tested positive for the virus. In 2006, Washington State had its first human cases when West Nile virus fever was diagnosed in residents of Pierce County (2 cases) and Clark County (1 case). In 2007, there were eight horses, one bird and one dog reported positive in Yakima County. There were no West Nile virus cases reported in King County.
West Nile virus can infect humans, birds, mosquitoes, horses and other animals, although disease is rare in dogs and cats. Wild birds become infected with West Nile virus and carry the virus in nature. Mosquitoes become infected after feeding on infected birds. People bitten by a mosquito carrying West Nile virus may have no symptoms at all or they may become ill with symptoms ranging from mild to severe. The less serious form is called West Nile fever, a flu-like illness that may last from a few days to several weeks. In the more severe forms, West Nile virus affects the nervous system causing swelling and inflammation of the brain or covering of the spinal cord (called neuroinvasive disease) and may result in paralysis and death.
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